In recent years, advances in technology, as well as ever evolving tastes in style, have led to substantial changes in the design of automobiles. One of the changes involves the power usage and complexity of the various electrical systems within automobiles, particularly alternative fuel vehicles, such as hybrid, electric, and fuel cell vehicles.
In most hybrid vehicles, energy storage devices, such as capacitors, are often used to improve efficiency by capturing energy within the powertrain system or supplying additional power during periods of operation when a primary energy source cannot supply the required power quickly enough. For example, regenerative braking may be used to capture energy by converting kinetic energy to electrical energy and storing the electrical energy in a bank of capacitors for later use. In order to accommodate high-voltage operation within automobiles, capacitor banks or supercapacitors are often used because they have the ability to quickly store energy and can be discharged at a much higher rate than other energy sources. However, capacitors may retain a charge after power is removed from a circuit or an automobile is turned off. Therefore, high-voltage capacitors should be properly discharged after turning off a vehicle or before accessing the equipment housing the capacitors.
Discharging a capacitor is typically accomplished by placing a discharge or bleed resistor across the capacitor or bus terminals in parallel. In addition to requiring additional components, these designs also require discharge resistors with the ability to handle high average power dissipation. These resistors generally occupy a larger surface area and often require additional harnesses, connectors, and heat sinks, which prevent the discharge resistors from being built on a circuit board. In addition to the increased spatial requirements, these discharge circuits are not utilized during normal operating modes.